1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures.
2. Description
Liquid crystals have recently gained considerable importance as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, the deformation of aligned phases (DAP type), the Schadt-Helfrich effect (rotation cell), the "guest-host effect" or a cholesteric-nematic phase transition.
The customary, static operation of liquid crystal indicating devices has in the past been replaced to an increasing extent by the so-called multiplex control. In this case there is mainly used an amplitude-selective multiplex procedure, whereby, however, by the procedures usually used in general only multiplex ratios of about 1:8 to 1:10 have been attained. In order to achieve higher multiplex rates in the control of liquid crystal displays, a two-frequency matrix addressing procedure has therefore been proposed (e.g. German Offenlegungsschriften Nos. 2 856 134 (Great Britain Pat. No. 2 013 014) and 2 907 940 (Great Britain Pat. No. 2 020 075).
In the case of this two-frequency procedure there is made use of the fact that the dielectric anisotropy of liquid crystals, which have upon application of a low-frequency voltage a positive anisotropy of the dielectric constants (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. &gt;O, .epsilon..sub..parallel. signifying the dielectric constant along the longitudinal axis of the molecule and .epsilon..sub..perp. signifying the dielectric constant perpendicular thereto), is negative in the case of high frequencies. This effect has been ascribed to the hindering of the rotation of the long axis of the molecule round the short axis of the liquid crystal molecule [M. Schadt, Mol. Cryst. Liq. Cryst. 66 (1981) 319-336]. In contrast to .epsilon..sub.81, in the case of .epsilon..sub.195 dispersion effects only appear in the microwave range because of the barely hindered rotation of the molecule around its longitudinal axis. In the frequency range which is of interest here .epsilon..sub..perp. is therefore constant, while .epsilon..sub..parallel. and consequently also .DELTA..epsilon. are frequency-dependent. The dielectric relaxation frequency, at which .epsilon..sub..parallel. =.epsilon..sub.195, is denoted in the technical terminology as the "cross-over frequency" (f.sub.c). The most common nematic liquid crystals generally have cross-over frequencies of about 100 kHz and above at room temperature.
For the operation of an indicating device according to the two-frequency procedure there are used two alternating current sources, whereby the frequency of one of these sources must lie above the cross-over frequency and the frequency of the other source must lie below the cross-over frequency. Moreover, the voltage ratio of the signals for the on-condition and the off-condition must lie above a certain value. The greater this voltage ratio is, the more lines can be portrayed, i.e. the greater is the multiplex rate.
In addition, the two-frequency procedure offers the advantage that not only the switching-on process, but also the switching-off process can be influenced directly by the application of a corresponding alternating voltage, whereby an acceleration of the switching-off process is achieved. For example, in the case of a liquid crystal indicating element with a twisted nematic structure (rotation cell) the homogeneously orientated liquid crystal can be aligned in the field direction by the application of a voltage of low frequency (f&lt;f.sub.c) and can again be converted into the twisted, homogeneous orientation by the application of a voltage of high frequency (f&gt;f.sub.c).
Furthermore, with liquid crystal cells, which are based on guest-host effects [Applied Physics Letters 13 (1968) 91; J. Appl. Phys. 45 (1974) 4718 inter alia], there is possible with the use of liquid crystals with positive dielectric anisotropy in general only the reading of colourless image elements on a coloured background, since the colouring substances which are usable mainly exhibit positive dichroism. By homeotropic wall orientation and control according to the two-frequency procedure there can now also be produced with such liquid crystals coloured image elements (homogeneously orientated by the application of a voltage of high frequency) on a colourless background.
The two-frequency procedure has, however, the disadvantage that the energy consumption is high, since not only the amplitude of the applied alternating voltage, but also the frequency are high. In order to reduce the energy consumption, it is therefore important that the operating voltage can be maintained low. The cross-over frequency should for the same reason lie relatively low (thereby the capacitative losses are small).